For all the fanfare around personalized medicine, the idea has been fairly slow to take off.

Boosters have said if doctors had a patient’s DNA information it would be revolutionary: They could look for genetic risk of certain diseases or mutations that determine whether certain drugs are likely to work or not.

The cost of DNA sequencing keeps falling, yet genetic sequencing for medical use isn’t commonplace. In the meantime, doctors can, in theory, consider a patient’s race in anticipating health conditions that could pose a higher risk.

But a study published recently in PLoS One suggests that using race as a stand in for truly personalized genetic information may not work, especially in the most diverse cities (like New York and Los Angeles) that attract immigrants from around the world.

There’s a lot of genetic variability among races. And the genetic risk profiles for many people with mixed ancestry don’t fit neatly into any category.

Researchers at the Mount Sinai Medical Center sequenced the DNA of nearly 1,000 local participants who identified themselves as European American, African-American, or Hispanic to determine their genetic make-up, or genotype.

In part, they wanted to see if the conventional race labels people use to identify themselves would be useful for doctors looking for genetic risk patterns in the community.

Many New Yorkers who self-identify as Hispanic originally came from the Caribbean but genetically are closer to African-Americans than immigrants from Mexico or Central America, the study found. So while both groups might call themselves Hispanic, their risk of disease will be very different.

“The great variability in those populations means the genomic risk of something like Body Mass Index may be valid for certain individuals and not others,” Erwin Bottinger, a study author and director of the Bronfman Institute at Mt. Sinai, told Shots. “That requires much more detailed analysis of genetic background of local communities.”

This means that doctors practicing in places like New York shouldn’t assume anything about disease risk in their community. “It’s ultimately much more important to focus on the individual,” says Bottinger.

Bottinger and others at the Bronfman Institute are now looking at hundreds of genetic markers for major diseases, such as heart disease, kidney disease, liver disease, and diabetes, in the Mt. Sinai patient community. But the end of the year they plan to have genetic info for 20,000 patients in a Biobank, which they say will help them predict disease risk for the community.